Method for the preparation of nanostructures and nanowires

ABSTRACT

A process for producing porous nanostructures suitable for the manufacture of nanowires and processes for manufacturing of the nanowires by using said nanostructure are disclosed. The process for obtaining the nanostructure of the invention comprises the steps of cleaning and polishing the surface of an aluminum metal substrate, forming a porous oxide layer bearing nanoholes on said aluminum substrate, immersing the porous structure into a basic zincate solution for etching the bottoms and walls of the nanoholes and depositing a thin and substantially pure Zn film extending from the bottom of the nanoholes through the aluminum substrate. Nanowires can be fabricated by the step of utilizing so obtained etched nanostructure as an electrode and subjecting the same to a metal electro-deposition operation.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an improved method for producing nanostructures bearing regular arrays of pores, and method of electrochemical metal or alloy deposition within said pores in order to form nano scale patterns and nanowires. The present invention is characterized by the principle of etching pore walls and bottoms by using a unique solution and making the same electro-conductive by forming a Zn film extending from inner walls of the pores through the substrate layer. The solution of the invention enables to provide the metal deposition with simplified and efficient electrolytic plating processes.

BACKGROUND AND SUMMARY OF THE INVENTION

Nanostructures, which consist of a porous alumina oxide film on a substantially pure substrate are conventionally produced according to a number of well known methods such as electrochemical anodization of aluminum or various lithographic techniques. Also a number of methods have been developed in the past years for providing the regularity of the pores within the oxidized alumina layer which is formed after the pre-treatment of the conductive or semi-conductive substrate. For the purpose of obtaining nanowire and nanopatterned functional surfaces, porous material as described above is subjected to the electrochemical deposition of certain metals within said pores. So obtained structures have wide applications in electronics, optics, photocatalysis, catalysis, photovoltaics, magnetics and superconductivity.

Oxide films can be formed with the traditional techniques such as electrolytic or non-electrolytic processes, sol-gel or chemical vapor deposition. Said pores on the aluminum oxide films produced by anodization are mostly desired to be regular arrays in the form of hexagonal honeycomb structure, and said oxide layer is constituted by a barrier layer and a porous layer. The porous layer is widely used for the production of nanowires and nanopatterned surfaces, and has a particular importance with respect to the nanostructure applications for use as a pattern. Various acidic solutions such as sulfuric acid, oxalic acid, phosphoric acid and chromic acid are used in the state of the art for the formation of nanoholes.

Formation of nanowires within each nanohole is carried out for instance by way of removing the substrate and barrier layers from the aforementioned porous structure and attaching the same to an electrode for electrochemical deposition of a definite metal inside the nanoholes. This is because the barrier layer is not substantially conductive for carrying out the conventional electrochemical applications. Such an application has been disclosed with U.S. Pat. No. 6,139,713 (Masuda et al.), wherein aluminum and barrier layers are removed for obtaining solely anodized alumina film having through holes. Accordingly, aluminum plate is selectively etched by using a saturated solution of corrosive sublimate (HgCl₂) or a saturated methanol solution of Br₂ to remove aluminum. Next, the barrier layer is removed using phosphoric acid or the like in order to obtain a through hole membrane. Said membrane can be used not only as a filter but also as a starting structure to be used to manufacture a regular structure of a metal or semiconductor such as nanowires. The process provides an effective way to obtain a nanostructure for use as a filter, however the process is rather complicated to operate and is not cost effective in terms of obtaining nano scale wires within the apertures of the nanoholes.

On the other hand, formation of a porous layer on any substrate may also be conducted by support of the mechanical ways such as nano-imprinting techniques in which a mold of recess/projection structures is directly pressed onto the surface of an object in order to obtain regular projections having holes in dimensions of nanometers. This lithographic technique can be used for later on manufacturing of nanostructures such as integrated circuits and microdevices as disclosed for instance in U.S. Pat. No. 5,772,905 (Chou). However, it is well known in the art that such lithographic applications damage the nanostructure in an undesired way and change the structural properties of the material making the same relatively unsuitable for many applications such as nanowires production.

Moreover, U.S. Pat. No. 5,747,180 (Miller et al.) discloses a method for manufacturing nanostructures and depositing certain metals within said structures in order to form quantum wires. According to an embodiment of the disclosure, preferred nanostructures are fabricated by way of selecting a semiconducting substrate, depositing a layer of metal on the substrate, electro-polishing the layer of metal so as to form pits therein, anodizing the metal layer so as to convert substantially all of the metal layer to an oxide layer containing pores, and depositing material in the pores. In order to form semiconductor compounds like CdS and ZnS in the pores, anodized aluminum is immersed into a solution of CdSO₄ and ZnSO₄ wherein the cation Cd²⁺ or Zn²⁺ reacts with S²⁻. The porous structure including the semiconductor compounds in the pore bottoms is not suitable for using the whole structure as an electrode for manufacturing the nanowires with any method of electrochemical treatment.

Further references indicating the well known fashion of etching either the substrate and barrier layers, as for instance EP 1378487, or etching of the pores have also been available in the art, however, a method comprising the basic idea of etching of the pore bottoms in order to penetrate through the aluminum oxide layer and reach the electro-conductive aluminum metal layer by means of an etchant solution which forms a film of an electro-conductive metal have never been proposed in the art.

In light of the prior art references cited herein as well as the unwritten practice of the technical field, the need for eliminating those problems mentioned above still exists. The present invention solves the problems mentioned above by an improved method of electrolytic deposition of metals or alloys within a porous structure. Said electrolytic way of deposition comprises the steps of anodizing a metal substrate in order to form a porous oxide layer, etching the pore bottoms with the unique zincate solution of the invention in order to obtain a thin zinc film in the inner wall and bottom portions of the nanoholes and making the same electro-conductive, utilizing the whole structure as an electrode and conducting the electrochemical deposition of certain metals, preferably nickel within said pores. The electrolytic process disclosed with the present invention does not require removal of the substrate and barrier layers for the electrochemical treatment since the pore bottoms of the oxide layer reaches the electro-conductive substrate and enables to use the whole structure as an electrode with a novel and inventive approach. Zincate solution of the present invention can also be used for electroless deposition of metals or alloys within the porous structure for obtaining nanowires or nanopatterned functional surfaces. The methods hereby presented provides a practical, cost effective and easy to implement way of fabricating nanowires as would readily be appreciated for those skilled in the art.

Further details and preferred embodiments of the invention with reference to the drawings are given in the following detailed description.

OBJECTS OF THE INVENTION

The present invention relates mainly to a method of obtaining a porous nanostructure suitable for use in fabrication of nanowires and a method of manufacturing nanowires and nanopatterned functional surfaces using said nanostructure.

A first object of the present invention is to provide a method for the production of a nanostructure suitable for manufacturing nanowires and nanopatterned functional surfaces, wherein the need for removal of the aluminum substrate and aluminum oxide layers for the electrochemical treatment is eliminated by means of etching the pore bottoms and making these portions electro-conductive.

Another object of the present invention is to provide a method for producing a nanostructure wherein pore bottoms are made electro-conductive with a unique etchant solution which forms a film of a pure metal extending from said pores through the electro-conductive layer.

A further object of the present invention is to provide a method for the fabrication of the nanowires and nanopatterned functional surfaces, wherein a unique etchant solution etches the inner walls and bottoms of the pores to reach the electro-conductive layer and accumulates therein, consequently eliminates the need for separate steps of etching of pore bottoms, deposition of a metal film and pore enlargement.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an ideal honeycomb like hexagonal porous structure formed after the anodization of a metal substrate;

FIG. 2-a, 2-b and 2-c are the SEM micrographs of the surface of a metal sheet polished with acid, thermal treatment and a combination of the thermal treatment and electro-polishing respectively;

FIG. 3 is a schematic view of an aluminum structure showing the movement of ions during the anodization procedure in order to form a porous oxide layer;

FIG. 4-a, 4-b and 4-c are the SEM micrographs of the surface of a metal sheet anodized in the presence of sulfuric acid, oxalic acid, phosphoric acid respectively;

FIG. 5 shows a cross sectional view of an ideal porous structure wherein aluminum substrate layer and aluminum oxide layer are explicitly shown;

FIG. 6 is a cross sectional view of the porous oxide layer wherein the pore bottoms are etched by using the unique solution of the present invention;

FIG. 7-a and 7-b are the SEM micrographs of the surface of the nanostructure treated with the unique zincate solution of the invention for durations of 60 s and 30 respectively;

FIG. 7-c is the SEM micrograph of the surface of the nanostructure prior to the treatment with the zincate solution of the invention;

FIG. 8-a and 8-b are the SEM micrographs of the surface and cross section of a nanostructure anodized in the presence of a sulfuric acid electrolytic solution;

FIG. 9-a and 9-b are the SEM micrographs of the surface and cross section of a nanostructure anodized in the presence of an oxalic acid electrolytic solution; and

FIG. 10-a and 10-b are the SEM micrographs of the nanowires formed in 2 A/dm² and 0.2 A/dm² current density values respectively.

FIG. 11-a and 11-b are the SEM micrographs of the nanowires formed within the porous nanostructures which are treated with the zincate solution for 30 s. and 60 s. respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the cost effective and uncomplicated production of the nanowires as well as the production of the nanostructures in which such nanowires are formed by way of the electrochemical deposition of a desired metal. In a preferred embodiment of the present invention, the metal deposited within the pores for the formation of the nanowires or nanopatterned functional surfaces is nickel, however, one skilled in the art may appreciate that said metal can also be any metal, alloy or compound suitable for use in the nano scale implementations such as Fe and Co or mixture thereof.

During the course of the anodization of an aluminum substrate, an oxide layer, namely a barrier layer is formed on the outer surface. If the electrolytic solution of the anodization process is capable of dissolving the barrier oxide layer, a porous layer forms on the outer surface with regular or irregular porosity due to the local dissolution of the oxide layer. Porous layer is mostly desired to have regular arrays of nanoholes such as a honeycomb like hexagonal structure as shown in FIG. 1.

According to the present invention, a nanostructure suitable for use as a pattern in manufacturing of nanowires is produced with a process comprising the general steps of;

-   -   polishing an aluminum substrate which is substantially pure and         suitable for nano implementations wherein said polishing step is         conducted by a chemical treatment or thermal treatment procedure         or electro-polishing or combination thereof,     -   anodizing the aluminum substrate in the presence of an acidic         electrolyte and dissolving the so formed porous layer in the         presence of an acidic electrolyte such as sulfuric acid, oxalic         acid or phosphoric acid,     -   applying a second anodization step in order to form the desired         porous layer having hexagonal arrays of nanoholes,     -   immersing the porous structure into a zincate solution for         etching the bottoms and inner walls of the nanoholes and         depositing a thin Zn film on all contact surfaces of the         nanostructure. Thereby, said structure is made suitable for         electrochemical deposition of various materials within the         porous structure for obtaining nanowires and/or nanopatterned         functional surfaces.

The process described above can be further continued with an additional step of electrochemical deposition of a metal or alloy in order to obtain said nanowires and/or nanopatterned functional surfaces. The zincate solution of the invention provides certain advantages for both electrolytic and non-electrolytic deposition procedures as per described in detail below. The nanostructure produced according to the steps of the given process is an alternative way over the prior art particularly for eliminating the need for the removal of the substrate and barrier layers prior to the electro-deposition of metals or alloys in an electrolytic deposition procedure.

Such an electrolytic process enables to utilize the whole structure as an electrode since pore bottoms are made electro-conductive with substantially a pure Zn film.

Accordingly, deposition of a metal or alloy through the nanoholes of the porous layer can be conducted by an additional deposition step wherein the whole structure mentioned above is utilized as the cathode of electro-deposition system. According to a preferred embodiment of the invention, electrolytic solution of this electro-deposition step can be a mixture of NiSO₄ and H₃BO₃ in any proportion and the metal deposited through the nanoholes can be Ni, Fe or Co.

Surface polishing of the aluminum substrate as per mentioned in the first step of the process given above has a particular importance for the successful formation of a uniform porous layer in the form of regular arrays of nanoholes. Chemical treatment of the metal substrate surface can be conducted for example by sequential treatment steps of NaOH and HNO₃. Thermal treatment is another way of pre-treatment of the substrate to be anodized. In such a procedure, substrate is immersed into oil and cleaned with an appropriate acid. Then said metal substrate is subjected to a heat treatment at a temperature around 450-550° C. in the presence of an inert gas atmosphere after which the grain size extends and becomes homogenous through the substrate structure. Electro-polishing is relatively an advanced way of surface polishing which is mostly preferred in sensitive nano implementations rather than the previous methods or it is conducted in combination with the previous methods. Exemplary SEM analysis results of the treatment techniques given above are shown in FIGS. 2-a, 2-b and 2-c, wherein FIG. 2-a shows the surface of a metal sheet cleaned with acid, FIG. 2-b shows the surface of a metal sheet subjected to the thermal treatment and FIG. 2-c shows the surface of a metal sheet subjected to thermal treatment and electro-polishing. Apparently, the metal surface treated with a combination of thermal and electro-polishing treatment steps provides better surface properties with improved smoothness.

Anodization of the metal as per mentioned in the second and third steps of the process given above is conducted for the purpose of obtaining an oxide layer which is constituted by a barrier layer and a porous layer having regular arrays of nanoholes. Two step anodization process provides the ideal hexagonal porous structure as per described by Masuda, H. and Fukuda, K., 1995. Ordered Metal Nanohole Arrays Made by a Two-Step Replication of Honeycomb Structures of Anodic Alumina, Science, 268, 1466-1468.

During the anodization steps mentioned above, an oxide layer consisting of a barrier layer and a porous layer grows up in the presence of an acidic aqueous solution according to the following general oxidation reaction. Also the ion movement through the structure is schematically shown in FIG. 3;

2Al+3H₂O→Al₂O₃+3H₂  (I)

The oxidation reactions of the anode belonging to the general reaction of (I) are given below;

2Al⁺³+3OH⁻→Al₂O₃+3H⁺  (II)

2Al⁺³+3O²⁻→Al₂O₃  (III)

If the electrolyte is not capable of dissolving said oxide layer, barrier layer forms without the porous layer. Therefore, the electrolytic solution chosen for the formation of the nanoholes within a porous layer has particular importance for obtaining a nanostructure suitable for the nanowire production. The acidic solution is preferred to dissolve the oxide layer substantially slower than the formation rate of the metal oxide and is also preferred to provide the local dissolution of the oxide layer in order to form the nanoholes in accordance with the present invention. Said electrolytic solution may comprise the well known acids such as sulfuric acid, oxalic acid, phosphoric acid and chromic acid. FIG. 4-a shows the porous structure formed in the presence of sulfuric acid, FIG. 4-b shows the porous structure formed in the presence of oxalic acid and FIG. 4-c shows the porous structure formed in the presence of phosphoric acid. Apparently, maximum pore size is obtained with phosphoric acid and narrower pores are obtained in the presence of sulfuric acid. Acidic electrolyte selection can be made depending on the desired size of the nanowires. A cross sectional view of an ideal porous structure is given in FIG. 5.

Porous structure of the previous anodization treatment steps is immersed into the unique zincate solution of the present invention as per described in the fourth step of the process given above. Unique zincate solution of the invention preferably comprises Zn(OH)₄ ²⁻ in combination with any strong base such as NaOH, KOH, Ba(OH)₂, Ca(OH)₂, LiOH, RbOH, CsOH and Sr(OH)₂. The strong base in the solution behaves as an electron donor for the formation of the non-ionic Zn film in the pore bottoms. Sequential reaction mechanism of the film formation is given below;

-   -   Anodic reaction;

Al⁰+3OH⁻→Al(OH)₃+3e ⁻

-   -   Cathodic reaction;

Zn(OH)₄ ²⁻+3OH⁻→Zn²⁺+4OH⁻

Zn²⁺+2e ⁻→Zn⁰

-   -   Overall reaction for the zincating process;

2Al+6OH⁻+3Zn^(2+→)3Zn+2Al(OH)₃

One of the inventive features of the present invention appears in the use of said solution which simultaneously etches the pore walls of the nanoholes and forms substantially a pure metallic Zn film in the pore bottoms, thereby provides excellent electro-conductivity between said pores and the aluminum substrate. The porous layer in which the pore bottoms are etched with the above mentioned solution is schematically shown in FIG. 6. Time duration of the zincate solution treatment is one of the parameters directly affecting the pore size of the nanoholes. FIG. 7-a shows the nanostructure after 60 seconds of zincate solution bath, FIG. 7-b shows the nanostructure after 30 seconds of zincate solution bath and FIG. 7-c shows the nanostructure prior to the zincate solution bath treatment. Consequently, a nanostructure suitable for easy fabrication of nanowires is obtained right after the above mentioned pore widening procedure and formation of the Zn film in the pore bottoms.

Electrochemical deposition of any desired metal through the nanoholes for manufacturing nanowires is traditionally conducted by way of first removing the substrate and barrier layers and obtaining the porous layer alone, then attaching the porous layer to a conductive metal for carrying out the electro-deposition step as per discussed in the background of the invention. According to another inventive feature of the invention, etching the pore bottoms and the barrier layer through the conductive aluminum substrate and filling so formed apertures with substantially a pure Zn film makes the porous layer highly electro-conductive and eliminates the need for removing said substrate and barrier layers in order to apply the electrochemical treatment of the nanowire production.

Electrolytic solution of the anodization for the formation of an oxide layer can be an acidic aqueous solution containing 1% to 50% of an acid selected from the group consisting of sulfuric acid, oxalic acid, phosphoric acid and chromic acid. A 3% to 15% acidic (v/v) solution is preferred. Applied voltage through the electrochemical treatment may vary in the range of 5 V to 100 V depending on the electrolytic solution and the desired thickness of the oxide layer. A voltage ranging from 20 V to 60 V is generally preferred.

The zincate solution to which the samples of the invention are immersed, comprises Zn(OH)₂ and a strong base such as NaOH, KOH, Ba(OH)₂, Ca(OH)₂, LiOH, RbOH, CsOH and Sr(OH)₂. NaOH is particularly preferred. Zn(OH)₂/Base ratio may vary 0.1 to 1.8 (by weight) in said zincate solution bath, however, said ratio is preferably kept in the range of 0.4 to 0.8 by weight.

Electro-deposition of metals within the conductive porous nanostructure of the present invention can be carried out in the presence of a bath solution consisting of NiSO₄ and a weak acid such as H₃BO₃, H₃PO₄, CH₃COOH, HF, CH₃COCOOH and C₅H₅NCOOH. H₃BO₃ is particularly preferred. Weight ratio of NiSO₄/weak acid may vary in the range of 1 to 50, however, said ratio varies more preferably in the range of 5 to 8. Ni, Fe or Co can well be deposited within the nanoholes of the porous nanostructure by means of the anodization step of the process disclosed above. Applied current density values may vary in the range of 0.1 to 20 A/dm², more preferably 0.2 to 2 A/dm² during the metal deposition.

Further advantages and best mode of carrying out the present invention are apparent and more recognizable to those skilled in the art in light of the following examples.

Example 1 Preparation of the Oxidized Nanostructure Suitable for Metal Electro-Deposition

Aluminum sheets having the size of 7×4 cm and thickness of 0.5 mm were used as the substrates during the experimental procedure. The substrates used for this purpose were supplied from Assan Alüminyum San. A.S.-Istanbul Turkiye, as 1050 series aluminum sheets which have the following specific composition;

Al Fe Si Cu Mg Mn Zn V Ti Others 99.5 0.4 0.25 0.05 0.05 0.05 0.05 0.05 0.03 0.03 min max max max max max max max max max

Aluminum sheets are subjected to the cleaning steps prior to the anodization procedure. Sheets are immersed into a 40 g/L NaOH solution at 55° C. for 2 minutes and then washed with purified water. Samples are polished by way of immersing the same into a 15% (by volume) HNO₃ solution for 30 seconds and washed again with distilled water. Anodization experiments were conducted twice with a 72V-12 A DC source in a double walled cooling bath wherein the temperature was controlled with a thermostat having ±0.5° C. sensitivity. The current passing through the circuitry was measured with a GANZ HDA-2 apparatus which has the measurement range of 60 mA-6 A. Separate sheets of aluminum were anodized in sulfuric acid and oxalic acid electrolytic solutions in various voltage values and number of pores in unit area have been observed. Results were as follows;

Number of Voltage nanoholes/cm² Size of the Electrolyte (V) (×10⁹) Nanoholes (nm) 15% (w/w) Sulfuric 20 52 10-13 Acid solution at 18° C. 3% (w/w) Oxalic Acid 60 6 30-35 solution at 18° C.

FIG. 8-a and 8-b shows the surface and cross sectional SEM view of the nanostructure respectively, which is obtained with the anodization procedure in the presence of sulfuric acid solution. Also FIG. 9-a and 9-b shows the surface and cross sectional SEM view of the nanostructure respectively, which is obtained with the anodization procedure in the presence of oxalic acid solution.

After the anodization of the aluminum sheets the sample anodized in the presence of oxalic acid gave the larger pore size compared to the sulfuric acid. Larger pore size provides more tolerance to the pore enlargement as well as to etching the pore bottoms during the zincate solution treatment. Therefore, the aluminum structure anodized in the oxalic acid solution was chosen for the zincate solution treatment in the next step. Anodized sample was immersed into a zincate solution consisting of 354 g/L Zn(OH)₂ and 525 g/L NaOH for 30 and 60 seconds. Pore size of the nanostructure varied as follows;

Time duration of Pore Size Level the Immersion (sn) (nm) 1 0 30-35 2 30 60-65 3 60  95-100

Inner walls and bottoms of the nanoholes are etched during the zincate treatment wherein etching of the pore bottoms for making the same electro-conductive and also enlargement of the pore diameters were conducted simultaneously within this single step. A thin Zn film extending through the aluminum substrate layer forms in the pores after which the nanostructure becomes suitable to be utilized in the further steps of metal deposition for manufacturing nanowires.

Example 2 Metal Electro-Deposition and Manufacturing of Nanowires

The porous nanostructure which was made electronically conductive for 60 seconds in the previous example was subjected to an electrolytic metal deposition step wherein said structure was utilized as the cathode and Ni was utilized as the anode electrode. Electrolytic solution bath was containing 300 g/L NiSO₄ and 45 g/L H₃BO₃ and said solution was heated to 50° C. before starting the electrolytic process. Two pieces of nanostructure sheets were used in the process in order to observe the amount of metal deposition depending on the current density. One of the pieces was subjected to 2 A/dm³ and the other one was subjected to 0.2 A/dm² current densities for this purpose. Pieces were washed right after the electrolytic process and analyzed with Scanning Electron Microscope. Results are shown in FIGS. 10-a and 10-b wherein the nanowires formed in 2 A/dm² current density and the nanowires formed in 0.2 A/dm² current density are shown respectively.

Apparently, the basic zincate solution mentioned herein is the key element of the present invention since it initially etches the inner surfaces of the pores and then forms the crucial Zn film extending from the inner walls of the nanoholes through the substrate layer. Said film of Zn makes the pores electro-conductive for electrolytic metal deposition. In other words, the zincate solution of the present invention provides effective metal deposition in electrolytic deposition processes. The nanostructure obtained as in Example 1 can be subjected to an electrolytic deposition procedure with a quite simplified way as in Example 2 for manufacturing nanowires.

It is easy for the skilled person to recognize that such deposition processes can be used not only for manufacturing of nanowires but also for manufacturing a wide variety of nanopatterned functional surfaces and devices suitable for use for instance in electronics, optics, photocatalysis, catalysis, photovoltaics, magnetics and superconductivity. It is also apparent for the skilled person to carry out the invention without the anodization step of the process for pore formation; instead the porous layer can also be formed by means of the well known techniques, for instance lithographic methods.

Appended claims provided herewith disclose the subject matter of the invention. The examples and illustrative figures do not limit invention in any way. 

1. A process for manufacturing of nanowires comprising the steps of; cleaning and polishing the surface of an aluminum metal substrate, and then forming a porous oxide layer bearing nanoholes on the aluminum substrate, immersing the porous structure into a basic zincate solution for etching the bottoms and walls of the nanoholes and depositing a Zn film from inner walls of the pores through the aluminum substrate, wherein said solution comprising Zn(OH)₂ and a strong base selected from the group consisting of NaOH, KOH, Ba(OH)₂, Ca(OH)₂, LiOH, RbOH, CsOH and Sr(OH)₂, and utilizing the etched nanostructure as an electrode and subjecting the same to a metal electro-deposition process.
 2. A process according to claim 1, wherein the porous oxide layer is formed by means of two anodization steps for providing regular arrays of nanoholes.
 3. A process for producing a porous nanostructure according to claim 2 wherein said anodization steps are carried out in the presence of an acidic electrolytic solution selected from the group consisting of aqueous sulfuric acid, oxalic acid, phosphoric acid and chromic acid solutions.
 4. A process according to claim 3 wherein concentration of said acidic solution varies in the range of 1% to 50%, more preferably 3% to 15% (v/v).
 5. A process for producing a porous nanostructure according to claim 1 wherein the Zn(OH)₂/strong base ratio in the zincate solution varies in the range of 0.1 to 1.8, more preferably in the range of 0.4 to 0.8 by weight.
 6. A process according to claim 1 wherein the metal deposited within the pores of the nanostructure is selected from the group consisting of Ni, Fe and Co.
 7. A process according to claim 1, wherein the electro-deposition is carried out in a current density ranging from 0.1 to 20 A/dm², more preferably 0.2 to 2 A/dm².
 8. A process according to claim 1, wherein the electro-deposition step is carried out in the presence of an electrolytic solution comprising NiSO₄ and a weak acid selected from the group consisting of H₃BO₃, H₃PO₄, CH₃COOH, HF, CH₃COCOOH and C₅H₅NCOOH.
 9. A process for producing nanowires according to claim 8, wherein the ratio of NiSO₄/weak acid varies in the range of 1 to 50, more preferably 5 to 8 (w/w). 